This invention relates generally to a laser system and more particularly to a laser system for standoff detection of hazardous particles on a specimen.
Standoff detection of hazardous materials remains an important challenge. It has been difficult to accurately detect trace quantities of explosives in a non-destructive manner in a public space containing many background chemicals. Non-linear Raman spectroscopy in the form of coherent anti-Stokes Raman scattering (“CARS”) has been tried for enhancing the signal of spontaneous Raman emissions through coherent signal addition. A CARS experiment is disclosed in H. Li, D. Harris, B. Xu, P. Wrzesinski, V. Lozovoy and M. Dantus, “Coherent Mode-Selective Raman Excitation Towards Standoff Detection,” Optics Express 5499, Vol. 16, No. 8, (Apr. 14, 2008). The analyte, however, was deposited on an ideal and highly reflective polymeric surface, which would not be present for most real-world situations where scanned passengers are wearing clothing made of natural or synthetic fibers (collectively, fabric) or leather, and luggage is made of plastic, fabric or paper-cardboard, which significantly diffuse or absorb the reflective light.
Other Raman-based techniques are disclosed in U.S. Patent Publication No. 2013/0162994 entitled “Systems and Methods Providing Efficient Detection of Back-Scattered Illumination in Modulation Transfer Microscopy or Micro-Spectroscopy,” published to Xie et al. on Jun. 27, 2013, U.S. Pat. No. 7,826,051 entitled “Coherently Controlled Nonlinear Raman Spectroscopy,” which issued to Silberberg et al. on Nov. 2, 2010, and U.S. Patent Publication No. 2008/0170218 entitled “Ultra-Fast Laser System,” which published to Dantus et al. on Jul. 17, 2008, all of which are incorporated by reference herein. It is noteworthy, however, that these prior patent references employ separate pump and anti-Stokes laser pulses. Furthermore, it is noteworthy that paragraph no. 0049 of the Xie patent publication highlights the differences of stimulated Raman scattering (“SRS”) microscopy over the CARS approach of the Silberberg patent.
In accordance with the present invention, a laser system and method employ stimulated Raman scattering using a single main laser pulse and a delayed replica reference pulse. A further aspect calculates stimulated Raman loss and stimulated Raman gain from a reflected laser light scatter collected from the surface of common objects such as fabric or paper. In another aspect, a laser system receives a low energy portion of a spectrum of main and reference laser pulses with a first photodetector, receives a higher energy portion of the spectrum of the main and reference pulses with either the first photodetector or with an additional second photodetector, and uses a controller to determine a Raman active phonon transfer of energy manifested as an increase in the reflected laser scatter in a lower energy portion of the spectrum and/or a decrease in a higher energy portion of the spectrum. In yet another aspect, the controller automatically determines if a hazardous particle or substance, such as an explosive, is present on a specimen within three seconds and using a pulse energy greater than 10 nanoJoules. A further aspect uses a laser system on a specimen located at least 0.5 meter away from a transportation security checkpoint structure to which a laser and photodetector are mounted. In still another aspect, a reference laser pulse has a different vibrational selectivity from a main laser pulse, yet the energies and spectra of the main and reference pulses are essentially identical. Software operating a laser system is provided which can calculate stimulated Raman loss and stimulated Raman gain from collected reflected scattered light, while minimizing distortions and background noise, to determine if a harmful substance is present on a light diffusing and/or absorbing specimen.
The laser system and method of the present invention are advantageous over prior devices. For example, the present method and system are capable of identifying small trace particles of hazardous substances on a light scattering and/or absorbing specimen such as fabric or paper. Another advantage is that the present laser system, method and software can scan large areas greater than 0.3×0.3 meters with results determined very quickly. The present system is also advantageous in being able to at least initially identify the hazardous particle on the light diffusing/absorbing specimen with a single laser pulse emission (which is subsequently replicated into an additional reference pulse). Furthermore, another advantage is the low energy required to operate the present system and method, which prevent undesired ablation or destruction of the hazardous substance on the specimen. It is also noteworthy that the present system and method employ SRS, thereby detecting by monitoring changes in an incident laser spectrum, which is different than CARS which detects at new frequencies and uses a narrow band probe pulse. In other words, the present system does not employ CARS and does not employ Raman spectroscopy, therefore it needs no spectrometer. It also noteworthy that the present system and method are advantageously capable of measuring changes in either half or all of the reflected light spectrum, including stimulated Raman loss or stimulated Raman gain, unlike the Xie construction which cannot measure both. Additional advantages and features of the present system will become apparent from the following description and claims, as well as the appended drawings.